1. Field of the Invention
This invention relates to flow imaging cytometry in which a specimen such a blood or urine suitably stained is introduced to a flow cell to form a flat, sheathed flow within the cell, the sheathed flow zone is irradiated with strobe light, and a cell image obtained by a video camera is analyzed by image processor. More particularly, the invention relates to an automatic focal-point adjustment method used in such flow imaging cytometry.
2. Description of the Prior Art
A cell analyzing apparatus for imaging cells flowing in the form of a flat, sheathed stream and automatically classifying and counting the cells utilizing image processing technology has been disclosed in the specifications of Japanese Patent Application Laid-Open (KOKAI) No. 57-500995 (1982) and U.S. Pat. No. 4,338,024.
An automatic focusing method is also known in which the image of an object produced by a lens is formed in an automatically focused state at a prescribed position. For example, the specification of Japanese Patent Publication (KOKOKU) No. 42-14096 (1967) describes a technique which uses an array of photoelectric elements having a very small surface area. Differences in output between mutually adjacent ones of the photoelectric elements are totaled and the total value is maximized when focusing is achieved. This technique detects the maximum point. Also known are various evaluation relations for evaluating the definition of an image. For example, the specification of Japanese Patent Publication (KOKOKU) No. 58-41485 (1983) describes making use of a mean-square function.
Since cameras such as still cameras or video cameras deal with images in which there is almost no change from one frame to the next, performing a focal-point adjustment while moving lenses or a group of image pickup elements in very small amounts is comparatively easy. However, in capturing a cell image or the like in flow imaging cytometry, the image changes every frame (every 1/30 of a second), and the positions and number of cells that appear in each frame change vastly. Consequently, performing focusing by comparing the same image from one frame to the next while moving a lens or the like in minute increments is difficult. Accordingly, automatic focusing is not easy to achieve.